Effects of Y and Zr Dopants on Grain Boundary Structure in Creep Resistant Polycrystalline Alumina
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Effects of Y and Zr Dopants on Grain Boundary Structure in Creep Resistant Polycrystalline Alumina G. S. Cargill III, C. M. Wang, J. M. Rickman, H. M. Chan, and M. P. Harmer Department of Materials Science and Engineering Lehigh University Bethlehem, PA 18015, USA ABSTRACT Dopants Y and Zr at 100 ppm levels in high purity, micron grain-size polycrystalline alumina are mainly segregated to the alumina grain boundaries and strongly reduce high temperature creep. Information about this segregation has come from high resolution STEM composition mapping experiments. Information about local structural surroundings of the dopant atoms has come from EXAFS experiments, and information about local bonding of the dopant atoms has come from XANES experiments. Structural models for dopant grain boundary segregation provide a context for these experimental results and for effects of dopant incorporation on grain boundary mediated transport. Recent experimental and theoretical results are discussed in this paper. INTRODUCTION Dopants Y and Zr at the level of 100 to 1000 ppm have been found to enhance alumina creep resistance by two orders of magnitude, as shown in Figure 1 [1-4]. A detailed knowledge of the atomic structures of dopant segregated grain boundaries will aid understanding of the dopant effect and will advance computer simulation and modeling of phenomena related to dopant grain boundary segregation. X-ray absorption spectroscopy, including EXAFS and x-ray absorption near edge structure (XANES), is an element specific local probe which is sensitive to low concentration dopants. Generally speaking, EXAFS spectra provide information about the coordination chemistry of the probe atom such as average bond lengths and coordination numbers at the probe atom. XANES probes more directly the density of unoccupied electronic states. It is complementary to EXAFS and provides information on stereochemical features of the coordination polyhedron around the probe atom, including bonding configuration, ligand-field symmetry, and valency state. In this paper, we discuss our recent high resolution STEM, EXAFS, and XANES investigations of high purity alumina doped with Y2O3 at concentrations of 100 and 1000 ppm, and ZrO2 at a nominal concentration of 100 ppm, giving information about the atomic structural environment of grain boundary segregated Y and Zr [5, 6]. EXPERIMENTAL PROCEDURES Details of the specimen preparations and experimental procedures have been reported previously [5, 6]. In brief, the materials were ultra high purity Al2O3 singly doped with ZrO2 and Y2O3 at a concentration of 100 ppm or 1000 ppm (Zr or Y to the Al ration). The doped
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Figure 1. Comparison of the creep behavior of undoped and 100 ppm Y doped alumina. powders were vacuum hot-pressed using graphite dies at 1450 °C and 45 MPa for 30 min. To vary the grain size, the as-hot-pressed materi
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